This invention relates to a method of producing a reflective mask for use in optical exposure in a semiconductor process and a method of producing a semiconductor device.
Recently, in the semiconductor industry, an EUV (Extreme Ultra Violet) lithography, which is an exposure technique using EUV light, appears promising following the reduction in size of a semiconductor device. It is noted here that the EUV light means light of a wavelength band within a soft X-ray region or a vacuum ultraviolet region, specifically, light having a wavelength of about 0.2-100 nm.
As a mask used in the EUV lithography, a reflective mask for exposure is proposed, for example, in Japanese Patent Application Publication (JP-A) No. H08-213303 (Reference 1). The reflective mask comprises a substrate, a reflective multilayer film formed on the substrate to reflect exposure light, and a buffer layer formed on the reflective multilayer film. On the buffer layer, an absorber film for absorbing the exposure light is formed as a patterned film. In an exposure apparatus using the reflective mask, light incident to the reflective mask is partially absorbed at a part where the absorber film is present and is partially reflected by the reflective multilayer film at another part where the absorber film is not present. The former part and the latter part may be referred to as an absorbing region and a reflecting region, respectively. An image formed by the light that is reflected by the reflective multilayer film is transferred onto a semiconductor substrate through a reflection optical system. Herein, the buffer layer serves to protect the reflective multilayer film when the absorber film is patterned by dry etching or the like in a mask production process. In order to increase the reflectivity for the exposure light, the buffer layer formed on the reflecting region (i.e., the part where the absorber film is not formed) of the mask is generally removed after the absorber film is patterned. As a result, the reflective multilayer film is exposed in the reflecting region.
As the above-mentioned reflective multilayer film, a multilayer film obtained by alternately laminating Mo layers and Si layers each having a thickness of several nanometers is known as a film adapted to reflect the EUV light having a wavelength of 13-14 nm.
The buffer layer is preferably made of a material which has high etch selectivity to the absorber film and which assures a smooth surface. As the material of the type, Japanese Patent Application Publication (JP-A) No. 2002-319542 (Reference 2) proposes a material containing Cr as a main component.
In Japanese Patent Application Publication (JP-A) No. 2002-122981 (Reference 3), it is proposed that a protection film formed on the reflecting region (i.e., the part where the pattern of the absorber film is not formed) of the mask is not removed after patterning of the absorber film but is used to increase the reflectivity for the exposure light.
In production of the reflective mask described in References 1 and 2, the buffer layer formed on the reflecting region of the mask is removed after the absorber film is patterned. In this case, the Si layer is generally formed as a topmost layer of the reflective multilayer film for the purpose of protection. This is because, in the above-mentioned reflective multilayer film comprising the Mo and the Si layers alternately laminated, Mo is more easily oxidized. Therefore, the buffer layer is formed on the Si layer as the topmost layer. For example, if the buffer layer is made of a material containing Cr as a main component and etched by the use of a gas containing chlorine and oxygen, the etch selectivity to the Si layer is as large as 20 or more. In this event, the buffer layer is patterned without no substantial reduction in thickness of the Si layer.
However, according to the inventor's study, it has been found out that, in the above-mentioned technique, a thin deposit of oxide is produced on the surface of the Si layer as the topmost layer of the reflective multilayer film. This is presumably because reaction is caused between oxygen and the Si layer at the top of the reflective multilayer film or an Si-based material within a processing chamber since oxygen-containing plasma is used upon removal of the buffer layer. Further, it has been found out that the deposit of oxide decreases the reflectivity of the reflective multilayer film. The thickness of the deposit of oxide is different depending upon the etching condition or the like but is generally equal to about 2 nm, as confirmed by low-angle X-ray diffraction or the like. It has been found out that the reflectivity is decreased by about 3%.
On the other hand, in Reference 3, the protection film formed on the reflective region of the mask is not removed after the absorber film is patterned. In this case also, it has been found out that, depending upon the etching environment upon patterning, a thin deposit of oxide, silicide, or silicon oxide is produced on the surface of the protection film. It has been found out that the thin deposit of oxide, silicide, or silicon oxide decreases the reflectivity of the reflective multilayer film by about 5%.
Heretofore, such deposition of oxide, silicide, or silicon oxide and resultant decrease in reflectivity of the reflective multilayer film are not known and, as a matter of course, no countermeasure has been taken.